What pressure do you have the front tires set at? That can be a huge factor. Mine were close to 50 psi straight off the lot, but that's not near enough. I run 65 in the fronts and that seems about right for max wear. I went 9k before the first rotation and at that point the rears needed relief from my right foot more than the fronts did!
Some people say that 5k is the ideal time to rotate tires on a 4wd. With bad cupping at 4k, I would say you either have the wrong pressure or have cheap tires or a combination of the above. Rotate them to the back and make sure you have enough psi in the front tires. What works best for tire wear isn't the most comfortable in the ride department.